Time-weighted, average (TWA) exposure measurements to toxic gases and vapors in the atmosphere is necessary to insure compliance with government occupational hygiene standards. One method to accomplish this involves drawing an air sample through a tube containing a sorbent, and analyzing the sorbent, usually by gas chromography, to make a quantitative determination of the sorbed, organic compounds. This technique is time-consuming, labor-intensive, and costly. More recently, colorimetric sampling devices have been developed, referred to in the art as "dosimeters" or "exposimeters," which are made available in the form of a badge, to be worn on the lapel of a worker, or mounted in a specific location, for a fixed time period, to enable a real-time measurement to be made of the exposure level of a gas or vapor pollutant in the atmosphere. Such devices are passive and depend on gas permeation and diffusion through the device to provide a real-time measurement of the exposure level to a pollutant gas. The measurement may be either a visual indication of the relative exposure level or a comparative measurement. Heretofore, a real-time, quantitative measurement of the time-weighted average exposure (TWA) was not available.
Several methods are known for estimating the time-weighted average (TWA) exposure and short-term exposure limit (STEL) to toxic gases and vapors in the atmosphere. These methods may be classified into the following major categories:
1. Methods which use the quality of a received colorimetric result (density, color, tone, shade, keen, or nuance), for comparison with a given color standard. The colorimetric devices in this category rely on the pollutant reacting chemically with reagent(s) dispersed on a suitable support to produce a color change. The estimation of pollutant is semi-quantitative, and it is reliable only in a comparably narrow exposure interval.
2. Methods which use the quantity of the received color expressed by area or length of stain. In general, the devices in this category consist of transparent tubes filled with a granular solid support impregnated with colorimetric reagent(s). Pollutants penetrate by diffusion through a diffusion retainer into the tubes and react with the colorimetric reagent(s). A colorimetric result forms as a length of stain proportional to the total exposure value of the concentration multiplied by the exposure time, that is, CT value. This provides a simple visual measure of exposure by comparing the length of stain to a fixed scale. However, the colorimetric result is nonlinear, and the response time changes with the length of stain.
3. Methods which rely upon the degree of gas permeation and/or gas diffusion to control the degree of gas penetration through a chromophoric reagent(s) to determine dosage exposure. These devices are referred to as "direct-read" because they visually provide a graded sensitivity of color detection for real-time measurement. One example of a direct-read, colorimetric dosimeter is taught in U.S. Pat. No. 4,478,792, in which a stack of porous sheets are impregnated with a reagent, causing the sheets in the stack to sequentially change color upon exposure to a gas pollutant in proportion to the degree of gas penetration. The degree of gas penetration is a function of the gas concentration in the atmosphere over a given time period. Another direct-read, colorimetric dosimeter is taught in U.S. Pat. No. 4,271,121, in which the indicator layer containing the colorimetric reagent is subdivided into a plurality of measuring fields. A graded sensitivity in color detection is achieved using a plurality of membranes of filter paper superimposed over the indicator layer in a staggered relationship, or by using different membrane thicknesses. The variation in membrane thickness or the staggered membrane arrangement varies with the diffusion resistance to the separate measuring fields in the indicator layer and, accordingly, varies the degree of gas penetration to each measuring field.
A more practical and useful direct-read, colorimetric exposimeter is taught in copending patent application entitled "Direct Read Colorimetric Exposimeter," Ser. No. 07/969,762, filed Oct. 30, 1992, now abandoned and assigned to the common assignee of the subject application. In this application, the disclosure of which is herein incorporated by reference, graded sensitivity to color detection is achieved by forming multiple measuring zones or fields, with each such zone or field hereinafter defined as a "diffusion cell." The exposimeter thus contains multiple diffusion cells, with each diffusion cell composed of a color indicator containing a chromophoric reagent which changes color when exposed to the polluting gas or vapor to be measured, and at least one gas-diffusion control member which provides a predetermined diffusion resistance to said polluting gas or vapor. Accordingly, each diffusion cell has a defined sensitivity to said pollutant and forms, in combination with the other diffusion cells in the exposimeter, an expanded dosage range for providing a graded measurement of color variation to most polluting gases.
The present invention is directed to a method and device for reading and analyzing a diffusion-cell, colorimetric exposimeter for making a quantitative determination of the time-weighted average (TWA) exposure to airborne gases or vapors.